1. Field of the Invention
The present invention relates to a printing apparatus to which a post-processing unit is connected, and in particular to paper feed control for controlling feeding of printing paper sheets in the printing apparatus.
2. Description of the Related Art
In recent years, printing systems including a post-processing unit, which is connected downstream a printing apparatus to perform various types of post-processing on printed printing paper sheets having subjected to printing at the printing apparatus, have been proposed. Examples of the post-processing performed by the post-processing unit may include stapling, punching, etc.
When post-processing is performed on printed printing paper sheets with this type of printing systems, the post-processing unit requires a time depending on the content of the post-processing. Therefore, when each printed printing paper sheet is passed from the printing apparatus to the post-processing unit, it is necessary to set the time depending on the content of the post-processing. However, if an excessively long time is set for passing each printed printing paper sheet from the printing apparatus to the post-processing unit, productivity may be lowered.
On the other hand, as the above-described printing apparatus with a post-processing unit connected thereto, a printing apparatus that is capable of duplex printing may be used. As the printing apparatus that is capable of duplex printing, for example, printing apparatuses where printing is performed on one side of each printing paper sheet fed from a paper feeding section of the printing apparatus, the one side printed printing paper sheet is inverted by an inverting section, and then printing is performed on the other side of the inverted one side printed printing paper sheet to achieve duplex printing have been proposed.
In order to improve productivity of this type of printing apparatuses for performing duplex printing, a method has been proposed where each one side printed printing paper sheet that has been inverted by the inverting section is inserted between printing paper sheets fed from the paper feeding section. Namely, when duplex printing is performed, a paper feeding time Tp for one printing paper sheet and a paper spacing interval Tg are set between the first printing paper sheet and the second printing paper sheet fed from the paper feeding section, and a one side printed printing paper sheet that has been inverted by the inverting section is inserted at a position corresponding to the paper feeding time Tp, as shown in FIG. 4.